1. Field of the Invention
The present invention relates to a toner carrier (hereinbelow referred to as a developing roller) and to an image forming apparatus which has a non-magnetic single-component developing apparatus that makes visible an electrostatic latent image formed on an electrostatic latent image holder (hereinbelow referred to as a photosensitive drum), using this toner carrier.
2. Description of the Related Art
In recent years, with the advance of office automation, considerable use is made of electro-photographic image forming apparatuses such as computer output terminal apparatuses, facsimile machines and photocopiers. In such apparatuses, a photosensitive drum is charged up by a charger and an electrostatic latent image is formed on the photosensitive drum by illumination with light. Then, toner (developer), which is controlled to be of uniform thickness on the developing roller, is made to adhere electrically to an electrostatic latent image on the photosensitive drum, thereby developing the image, which is then transferred to recording paper and fixed. After transfer, the residual toner on the photosensitive drum that was not transferred is recovered by a cleaner, and preparations are made for the next printing.
In this process, the recovered residual toner becomes spent toner, and is accommodated in a receiving box before being subjected to disposal treatment; however, since the spent toner is a powder, this disposal treatment presents a problem. It is therefore desirable, from the point of view of running costs and environmental protection that the spent toner should be used up or employed for recycling. However, the residual toner on the photosensitive drum after transfer is subject to toner quality variability such as for example of the toner particle size, content of external additives, and adhesion characteristics, and this may cause failure of electrical charging or non-uniform electrical charging.
Also, since, during transfer, the paper is in contact with or in proximity with the photosensitive drum, dust such as paper dust sticks to the photosensitive drum and is recovered by the cleaner in the same way as the residual toner; toner returned to the developing apparatus therefore undergoes severe deterioration which makes recharging difficult, resulting in problems such as a low degree of charging up, giving rise to contamination i.e. so-called "fog" (hereinbelow referred to as "fog") in the image background. Reuse is therefore difficult.
Methods of development employed in such an apparatus include: the single-component developing method, in which only toner constituting non-magnetic single-component developer is employed, and the dual-component developing method in which a carrier is employed in addition to the toner. The single-component developing method has the advantage that the construction of the image forming apparatus is simplified, since no particular care needs to be exercised regarding carrier deterioration or the mixing ratio of carrier and toner, since no carrier is employed.
Also, in the case where the single-component developing method is used, in contrast with the case where adhesion on to a magnetic roller is effected by employing a developing agent comprising a mixture of carrier and toner as in the dual-component developing method, since the single-component developing agent does not employ a carrier, adhesion is effected by conferring charge on the developing roller by forcibly charging up the developing agent.
To achieve this, toner of comparatively high volume resistivity is employed; if, for example, toner of 10.sup.10 to 10.sup.13 .OMEGA..multidot.cm is employed, it is necessary that this should be forcibly charged up to the prescribed polarity. A well-known conventional arrangement for achieving this is to confer frictional charging-up charge on the toner by a frictional charging member. As the frictional charging member, in the case of a toner layer thickness regulating blade, for example, a blade whose tip is constituted by a frictional charging member consisting, for example, of silicone rubber or polyurethane is employed; in the case of a roller whereby toner is supplied and recovered, for example, a roller constituted by a frictional charging member consisting of, for example, conductive silicone sponge or polyurethane sponge is employed. Using a developing roller that is in contact with both members, frictional charging-up of toner can be achieved with a construction of optimum simplicity and low cost.
Problems in the application of such a conventional developing roller are described below. In the contact developing method, in order to achieve effectiveness of the developing electrode and effectiveness of the developing bias, the developing roller is formed with a conductive resilient layer at the circumference of its shaft and, if necessary, it is desirable that bias voltage should be applied thereto. However, environmental variation (in particular, rise in temperature), etc., is produced by addition of plasticizers and/or softeners, etc., in order to achieve the prescribed resistance and/or hardness, and the phenomenon of bleeding of these additives occurs, causing contamination of the photosensitive drum.
Thus, when a conductive resilient layer such as rubber is formed on the developing roller, the problem arises that image loss on the printed surface is caused by contamination of the surface of the photosensitive drum due to migration of the softening agent, etc., from the conductive resilient layer to the photosensitive drum. In order to prevent contamination of the surface of the photosensitive drum, it is therefore necessary to provide a film consisting of a member that does not contaminate the surface of the conductive resilient layer, in order to prevent contamination of the photosensitive drum surface.
In order to solve this problem, use of surface films of conductive resilient layer member constituted by a resin coating is known. However, in the case of a resin film, there are problems in that the equipment may become unusable if peeling occurs if there is poor adhesion with the resilient layer and/or the wear resistance of rubber etc. is poor. Furthermore, if a hard resin layer is formed by addition of for example conductive carbon, the toner is melted by the heat of friction with the result that it fuses on to the surface of the developing blade or filming occurs due to adhesion thereof to the surface of the developing roller.
Also, the resin film of the developing roller has the function of a frictional charging-up member and, due to interaction with the toner, for example in regard to charging-up polarity and/or coefficient of friction etc., may give rise to toner of reversed charging polarity or may give rise to uncharged toner or non-uniformly charged toner, creating problems of fog.
Also, due to the different water-absorbing characteristics of resin members, there may be considerable fluctuations in electrical resistance, depending on the environment. That is, since the resistance becomes high at low temperature compared with normal temperature and at low humidity and contrariwise falls at high temperature and high humidity, the amount of charging up of the toner also becomes high under low temperature and low humidity and falls under high temperature and high humidity. If therefore the amount of charging up of the toner becomes too much higher than the prescribed amount of charging up, when the electrostatic latent image is made visible, this is done with a smaller quantity of toner causing a drop in density; if the amount of charging up of the toner is too much lower than the prescribed amount of charging, this gives rise to the problem of fog of the image background. Furthermore, if the distribution of the amount of charging up among the toner particles is widened, there is the problem that such drop in density or fog is rendered even more severe.
There is therefore an appropriate amount of charging for the toner and problems arise if it is higher or lower than this. Furthermore, it is important that the distribution of the toner particles should be narrow. Furthermore, in order to provide high picture quality in stable fashion, it is necessary to maintain the amount of charging and the charge distribution constant irrespective of the number of printed sheets and/or changes in the environment. To achieve this, it is necessary that the amount of frictional charging should not change, even if the environment or number of printed sheets etc. changes. A non-magnetic single-component developing roller must therefore confer a stable charge on the toner.